Random Early Detection for Congestion Avoidance Sally Floyd

Drop tail Gateways • End to End Congestion control may result in global synchronization.

Introduction. • RED provides congestion avoidance by controlling the queue size at the gateway.

RED Algorithm. • RED parameters – Minimum queue threshold (min. Q) – Maximum queue

RED Algorithm • For Each incoming packet – If Avg. Q <= min. Q

Average Queue Length • Avg. Q = (1 – weight) X Avg. Q +

Drop Probability • Temp. P = Max. P x (Avg. Q – min. Q)/(max.

More on RED • Function of Average Queue length – Small bursts go untouched

Advantages of RED gateways • Congestion Avoidance – If the RED gateway drops packets

Other Issues with RED • Fine tuning min. Q, max. P and weight needed

Slides: 10

Download presentation

Random Early Detection for Congestion Avoidance Sally Floyd Van Jacobson

Drop tail Gateways • End to End Congestion control may result in global synchronization. • The source takes action only after congestion actually occurs. • Droptail routers biased against bursty traffic. • Transient and persistent congestion not distinguished. • Gateway has no say in congestion control.

Introduction. • RED provides congestion avoidance by controlling the queue size at the gateway. • RED notifies the source before the congestion actually happens rather than wait till it actually occurs. • RED provides a mechanism for the gateway to provide some feedback to the source on congestion status.

RED Algorithm. • RED parameters – Minimum queue threshold (min. Q) – Maximum queue threshold (max. Q) – Average Queue length (Avg. Q). • This is calculated dynamically – Maximum drop probability (max. P) – Drop probability (P) • Calculated dynamically.

RED Algorithm • For Each incoming packet – If Avg. Q <= min. Q • queue packet – If min. Q <= Avg. Q < max. Q • Mark packet with probability P – If max. Q <= Avg. Q • Mark the packet

Average Queue Length • Avg. Q = (1 – weight) X Avg. Q + weight x curr. Q – 0 < Weight < 1 – curr. Q is the current queue length • Avg. Q may be calculated for each new packet arrival or at fixed time intervels.

Drop Probability • Temp. P = Max. P x (Avg. Q – min. Q)/(max. Q – min. Q) • P = Temp. P/ (1 – count. Temp. P) – Count is number of newly arrived packets queued. • This Extra step has been introduced to keep the drops evenly spaced in time.

More on RED • Function of Average Queue length – Small bursts go untouched – Affects long loads • Misbehaving users can be identified and severely penalized. Co-operating users get overall delays.

Advantages of RED gateways • Congestion Avoidance – If the RED gateway drops packets when avg. Q reached max. Q, the avg. Q will never exceed max. Q. • Appropriate time scales – Source will not be notified of transient congestion. • No Global Synchronization. – All connection wont back off at same time. • Simple • High link utilization • Fair

Other Issues with RED • Fine tuning min. Q, max. P and weight needed for optimum performance. • RED needs to be deployed at the edge of the network.